The present invention relates to a handoff (busy channel switching) method for a cellular digital mobile communication system constituted by small radio zones (cellular system).
A service area of the cellular system is divided into a large number of radio zones (cell sites), and identical frequencies are utilized again in remote cell sites to increase the number of mobile telephone sets per frequency. The number of potential users of mobile telephones is larger than the actual number of users covered by the existing systems. Therefore, strong demand has arisen for a large-capacity system. The size of the cell site may be reduced to increase the system capacity within the limited frequency band. For example, if the radius of the cell site is reduced to 1/10, frequency utilization efficiency can be increased to 100 times.
When the size of the cell site is minimized, however, the frequency of communication channel switching, i.e., a handoff is increased since a mobile station is moved from one site to another during communication. A handoff sequence of an existing system is described in Bell System Technical Journal Vol. 58, No. 1, January 1979, P. 65. Upon designation of a new cell site channel, a mobile station interrupts communication via the present channel, and the channel is then switched to a new channel. In this state, the mobile station performs a communication test and restarts communication on the new channel. According to this method, communication is interrupted for 0.7 to 0.8 sec during the communication test.
Since the communication is interrupted for 0.7 to 0.8 sec according to the conventional handoff method, communication quality is greatly degraded when the channels are frequently switched upon an increase in the number of cell sites.
In addition, in the conventional handoff method, detection of movement of a mobile station between cell sites is concentrated on a base station and a radio channel control station. When the size of the cell site is minimized, the handoff operation must be frequently performed, and a control amount is locally increased, thereby requiring a large-capacity apparatus.